1. Technical Field
A method for forming a floating gate in a flash memory device is disclosed capable of preventing a thickness of a buffer oxide film from being increased due to a first polysilicon film in a subsequent wall oxidation process, and which is capable of minimizing an attack of the first polysilicon film during a pad nitride film strip process.
2. Discussion of Related Art
In recent, a self-aligned shallow trench isolation (SA-STI) process has been used as a device isolation process in realizing a highly integrated NAND flash memory device. Accordingly, a floating gate is divided into a first polysilicon film and a second polysilicon film to form a stack structure. At that time, a buffer oxide film is deposited with a suitable thickness prior to depositing the first polysilicon film and a pad nitride film for the purpose of relieving a stress between the pad nitride film and the first polysilicon film. This buffer oxide film is used as a barrier film for relieving a stress between a pad nitride film and the first polysilicon film and preventing the first polysilicon film from being damaged in the pad nitride film strip process. Next, the buffer oxide film is removed in a pre-treatment cleaning process of a second polysilicon film and then a high-density plasma (HDP) oxide film is deposited and etched to form a slowness profile. However, as shown in FIG. 13, the first polysilicon film is oxidized during a subsequent wall oxidation process, thereby increasing a thickness of the buffer oxide film. Accordingly, as shown in FIG. 14, the buffer oxide film remains at an interface between the first polysilicon film and the second polysilicon film. Since the remaining buffer oxide film is completely removed in the pre-treatment cleaning process of the second polysilicon film, an over HF dip time is required for completely removing the buffer oxide film. In addition, a real thickness of the first polysilicon film is decreased to 10% to 20% of the thickness at the time of depositing the first polysilicon film. In oxidation of the first polysilicon film, oxidation degree of the wall oxidation process is different depending on magnitude of a critical dimension of the isolation thereof, whereby the thickness thereof increased at that time is also different. Accordingly, there is a problem that the buffer oxide film cannot uniformly remain over a whole surface of a wafer.